In a known form of apparatus for contact drying sheets of wood veneer, a pair of endless belts is mounted over and driven intermittently by guide rolls to advance a veneer sheet between said belts through the apparatus and also to allow heat from heating plates to be conducted through said belts to the veneer sheet to be dried. The heating plates are disposed above and below the belts, respectively, and the plate at least on one side is movable alternately into its operative or heating position then with the endless belts at a stop and away from said heating position then with the belts in movement for advancing the veneer sheet. Thus, the veneer sheet can be dried progressively by repeated application of heat through the belts while being forwarded intermittently therewith through the dryer. The structure of the dryer of this type is disclosed by Publications (or Kokoku) of Japanese Utility Model Application No. 59-34872 (1984) and of Japanese Patent Application No. 60-12547 (1985), both assigned to the same assignee as the present application, laid open to public opposition, respectively. Incidentally, the basic arrangement of the endless belts, guide rolls and heating plates with respect to the veneer sheets to be dried is shown in FIG. 1 accompanying herewith.
Though this type of veneer dryer is known to be advantageous in simplicity of construction, improvement of thermal efficiency and ease of control, as compared with a conventional veneer dryer of hot air type, it is not practically applicable to mass production of dried veneer.
For accomplishing an increase of production capacity in drying veneer sheet having a normal thickness with the above dryer using a pair of intermittently-running endless conveyer belts to an extent obtainable by conventional hot air type dryers, it would be necessary either to provide plural pairs of endless conveyer belts in a multiple-tier arrangement or to form a single tier of veneer veneer passage long enough to handle a number of sheets at a time. However, provision of the multiple tiers of veneer passage make it troublesome to handle the sheets at feeding and delivery sides of the dryer because it will have plural inlets and outlets for green and dried veneer, respectively. The use of the latter dryer having a long veneer passage requires accordingly a large space for installation of the machine. Therefore, application of such dryers is impractical to mass production of dried veneer in veneer of plywood mills of an ordinary size and their possible application is limited to drying extra-thick vener sheets which are difficult to be dried by hot air type dryer.
The above problems may be solved by providing guide rolls of plural pairs in such a staggered arrangement that the endless belts trained round such rolls may form a veneer passage having a series of rounded turns in alternating directions. It is known, however, that other problems arise in such an arrangement which seriously affect the smoothness and stability of movement of the endless belts, if they are made of a loop of thin metal plate having very little resiliency unlike, e.g. net of metal wires.
Because the application of heat to veneer sheet by heating plates is accomplished by conduction through the endless conveyer belts, it is advantageous to form the belts from a thin metal band having high heat conductivity and sufficient strength. As a disadvantage, on the other hand, such metal belt has very little resiliency. When a pair of such belts are moved in a winding course of alternating directions while being guided by rolls disposed in a staggered arrangement, the two belts run at different speeds round the periphery of a roll because of the difference in radius of curvatures between the two belts round the periphery of the roll, with the outer belt moving faster than the inner belt, which results in slackening of the belts. Furthermore, the inner belt at a curved turn is inevitably restricted or held down directly by the outer belt, or indirectly by way of a veneer sheet held between the belts. In addition, the belt movement through the alternate turns causes the belts to be bent in opposite directions alternately, and the restriction of one belt by the other and also the difference in speed round the peripheries of rolls between the two belts are varied similarly in an alternate manner. Though the endless belts are usually placed under a tension to pull in the slack produced therein, the endless belts of the above arrangement are unable to absorb such slack. It is because there exists an area in the veneer passage which is located between two guide rolls where one belt is held down by the other and, therefore, the slacking in the belt in that area cannot be taken up by the tension to the belt. Such slack in the belt tends to be increased with continuation of the movement of the belts along the winding passage, which affects seriously the smoothness and stability of belt operation. Such behavior of the endless belts made of a rigid material such as steel sheet has been observed by the experiments conducted by the present inventors. Thus, it is quite impractical to use a pair of such endless belts of the above arrangement for successfully transferring a veneer sheet in a contact-type veneer dryer.
For better understanding of the characteristics of metal endless belts used for the above conveying purpose, their behavior in operation will be described more concretely with reference to FIGS. 10 and 11 accompanying herewith. In the drawings, a pair of endless belts 21, 22 is provided to as to be moved round, while being guided by, rolls 22a, 21a, 22b, 21b and other rolls (not shown) which are located in a staggered arrangement, and a veneer sheet designated by symbol T is conveyed along a winding passage defined by the endless belts and having a series of turns in alternating directions, moving intermittently therewith while being held or sandwiched therebetween.
As the veneer sheet T is moved from the dash-line position round the periphery of the roll 21a to the solid-line position in the straight passage between the rolls 21a and 22b, there is produced a slack in the belt 22 because it has to move a longer distance round the periphery of the roll 21a than its counterpart 21 because of the reason stated previously. Since the belt 22 is held down by the other belt 21 round the rolls 22a and 22b, the slack produced in the belt 22 cannot pulled in by the tension applied thereto, but remains therein in the area between two such rolls 22a and 22b, as shown in FIG. 11.
The same is true of the other belt 21 which will have to move a longer distance than the belt 22 round the roll 22b. Consequently, a slack is produced in the belt 21 which cannot be taken up because of its restriction by the belt 22 in the area of veneer passage between the rolls 22b and 21b.
This phenomenon of slackening tends to become more remarkable when another veneer sheet is then present round the periphery of the roll 22a in addition to the sheet T shown round the periphery of the roll 21a. It is noted that the locations and amount of belt slackening are varied according to various influencing factors such as position of the rolls, relative position of one veneer sheet to another in the passage, coefficient of friction between the veneer sheet, rolls and belts, etc. Though stronger tension may be applied to either of the belts with an attempt to take up its slack, it will inevitably increase the restriction thereby of the other belt, which means that the slack in the latter belt has no way to be pulled in. In any way, it is unavoidable to have slacking produced in the endless belts, and thus impractical in the above staggered arrangement of drive rolls to allow the endless belts to move round such rolls without producing belt slackening which causes difficulty in feeding veneer sheets with smoothness. This has been verified by the experiments conducted by the present inventors.
Reference is then had to FIG. 12 showing an example of relative arrangement of endless belts to guide rolls, which offers an improvement over that described with reference to FIGS. 10 and 11. This arrangement includes a longer endless belt 31 trained around a plurality of guide rolls 31a, 32b, 31d, 31c and 31b which are disposed in a staggered manner, and a short endless belt 32 mounted over a pair of rolls 32a and 32b. As shown in the drawing, this configuration has only one location where the two belts are moved together round a roll, namely the roll 32b, and, therefore, any slack in the belt 31 caused by the difference in speed between the belts can be successfully absorbed by the tension applied to the belt, thus posing no fear of troubles that have been described previously. With this structure wherein the inlet for green veneer to be dried and the outlet for dried veneer are provide on the same side of the machine, or on the left-hand side as seen in FIG. 12, it offers a serious disadvantage in that the flow of veneer processing in the whole production line is reversed in this drying station, thereby making it difficult to install the machine in an ordinary streamlined production line. Though the apparatus can provide an advantage in terms of space factor, it is not so important in view of its disadvantage.
Then, reference is made to FIG. 13 showing a known arrangement of conveyer belts and their guide rolls, which is primarily designed for conveying sheet materials not limited to wood veneer. The apparatus includes plural pairs of guide rolls 41a, 42a, 51a, 52a, 61a, 62a (only one of each pair being shown), endless belts 41, 42, 51, 52, 61, 62 each trained over the rolls of each pair, plural sets of small rolls, each set including 71a, 71b, 71c and 72a, 72b, 72c, disposed adjacent to the peripheries of the rolls 42a and 52a, respectively, and short endless belts 71, 72, installed over the small rolls of each set and driven thereby at a surface speed which is slightly lower than that of the long endless belts. It is noted that similar sets of small rolls and short endless belts are provided at rolls (not shown) on the left end of the loops of endless belts 51 and 61, respectively. With this apparatus, although there is no problem associated with mutual interference or restriction between the endless belts, not only its structure becomes complicated by addition of small rolls and their associated short endless belts, but also part of the passage at large rolls, e.g. rolls 42a and 52a in FIG. 13, forms an open space where veneer sheets, if fed through the passage, in particular narrow sheets, tend to fail to be transferred smoothly, as indicated by irregularly bend sheets. Thus, this arrangement of endless belts and rolls cannot provide an effective solution and its application to a veneer dryer of the aforementioned type is impractical.
As it would be apparent from the foregoing description, it had been presumed difficult in a veneer drying machine to realize a usable arrangement of guide rolls disposed in a staggered manner and non-resilient metal endless belts trained over such rolls thereby to form a veneer passage having a series of turns in alternating directions. Attempts had been made to partially overcome this presumption by improving the arrangement. Such an improvement is shown by Japanese Patent Applications No. 61-237679 (1986) and No. 61-238644 (1986) for "Veneer Drying Apparatus", both assigned to the same assignee as the present application.
In the veneer dryer according to the Japanese Patent Application No. 61-237679, which is basically the same in respect of the arrangement of guide rolls and endless conveyer belts as the apparatus shown in FIG. 5, a pair of endless belts are trained around a plurality of staggered rolls to form a veneer passage comprising a combination of three straight, flat passages and two curved passages each located so as to connect the ends of any two adjacent straight passages. In other words, the apparatus features in that it forms a winding veneer route comprising top, middle and bottom straight passages where the veneer sheets are heated for drying while being advanced between the intermittently-moving endless belts and two curved passages, one located between the terminating end of the top heating passage and the beginning end of the middle passage and the other between the terminating end of the middle passage and the beginning end of the bottom passage.
On the other hand, the veneer drying apparatus according to the Japanese Patent Application No. 61-238644 differs from the above dryer in that an appropriate number of pairs of cooling plates are provided with at least one of such plates of a pair being movable toward and away from the endless belts and at least one pair of such cooling plates being located at the end of the veneer passage.
These dryers can be installed in a limited space and make it possible to accomplish smooth and stable feeding operation. With the latter apparatus having cooling plates, the veneer sheet can be hardened by cooling effect by the cooling plates and, therefore, deformation caused by drying can be forestalled successfully. Though both dryers could offer various advantages and be usable successfully in veneer and plywood mills handling a large volume of veneer, there has arisen a problem associated with maintenance of the endless belts within a permissible area for running track thereof during their operation.
In a combination having an appropriate number of rolls and an endless belts trained over such rolls under tension and adapted to run round the rolls while being guided thereby, it has been an usual practice for correction of the running track of the belts to provide a pair of detectors at the opposite lateral sides of the belt for monitoring the lateral displacement or swerving motion of the endless belt and to tilt the associated roll axially in response to a signal from the detectors so that the track of the endless belt may be corrected gradually by the difference in tension of the belt between its axial ends.
This method for correcting the running track of an endless belt which relies on the difference in tension of the belt created by axially tilting the roll is found ineffective in the veneer dryer of the above-mentioned type in which the metal endless belts tends to be temporarily shrunk or contracted locally due to their contact with green and hence cold veneer sheet just being fed into the dryer, and the location and coefficient of such contraction vary depending on various influencing factors which are difficult to predict, such as shape, moisture content, etc. of veneer sheets to be dried. As a result, the time of belt track correction tends to be delayed or even disabled, thereby causing the belts to move far away from the permissible area of belt track. According to the results obtained from our experiments, with the veneer drying apparatus according to the referred to Japanese Patent Application No. 61-237679 or No. 61-238644 wherein one of the endless belts of a pair is held down by the other at each curved turn and their tight and loose sides are reversed each time they are moved past a turn, it has been found that the amount and direction for correction of the belt track in relation to the axial displacement of the roll is varied unpredictably. Thus, it has been very difficult to secure stabilized movement of endless belts for a substantial period of time in operation with the above veneer dryer.